(REUTERS) – On a recent evening on Hungary’s border with Serbia, a fleet of police trucks raced along the dusty boundary.

A heat sensor trained on the razor wire fence had picked up migrants approaching, officers explained. The migrants, about two dozen in number, quickly split up and went back deep into Serbian territory, police said.

It was a scene that has played out many times in the months since Europe’s migrant crisis erupted.

“The human traffickers are never idle. Their night-vision drones watch our patrol system. They take advantage as soon as we redeploy our forces,” said police commander Zsolt Gulyas.